The invention relates to the field of optics and more particularly to optical ellipsometry-based methods and apparatus for measuring refractive index and optical absorption differences.
Optical ellipsometry-based methods for measuring refractive index and optical absorption of materials are known in the art and have been used to study a wide-variety of different materials. Prior art ellipsometry methods, however, tend to lack the signal-to-noise ratios necessary to detect changes in optical response from a few percent of one monoatomic or monomolecular layer, thus in general restricting their use to systems giving rise to changes greater or much greater than a few percent of one monoatomic or monomolecular layer. In cases of special forms of ellipsometry such as oblique-incidence optical reflectance difference and surface photo-absorption, prior art ellipsometry techniques teach the measurement of only one or one combination of the refractive index and the optical absorption coefficient rather than two, and so fail to realize the benefits of simultaneous determinations of refractive index and optical absorption coefficient differences. Prior art ellipsometry techniques also fail to provide a solution to the problem of determining a refractive index difference or an optical absorption coefficient difference between two samples under circumstances in which the signal-to-noise ratios of the first or the second order harmonic is inadequate because of environmental or instrument noise in these regions of the spectrum.
The present invention addresses these and other deficiencies of the prior art by providing methods and apparatus for determining refractive index and optical absorption coefficient differences with significantly improved signal-to-noise properties, for simultaneous monitoring of refractive index and optical absorption coefficient differences, and for obtaining higher order harmonic measurements.